The present invention relates to a method of producing a micro-porous membrane and to the membrane so obtained. More particularly, the invention is concerned with a diaphragm intended for use in electrolysis cells.
Although diaphragm-type cells have been known for many years, the effect of the diaphragm has not been clearly explained. The old explanation, according to which the diaphragm acts as a filter bed and according to which the flow of the brine and the migration of the ions can be calculated when the number of pores and their means diameter are known, appears to be an over-simplification as observed by J. S. Sconce in "Chlorine, Its Manufacture, Properties and Uses," American Chemical Society--Monograph Series No. 154, page 105, 1962.
In the absence of a satisfactory explanation, it has been found that such diaphragms should have quite conflicting properties. In particular, they should have a large proportion of cavities but at the same time they should possess adequate mechanical strength. Furthermore, their configuration should be such that good diffusion of the brine takes place, while at the same time the solutions in the anode and cathode compartments are sufficiently retained, the formation of chlorate in particular being avoided. Numerous methods have already been proposed for obtaining these diaphragms.
For some time the tendency has been towards diaphragms based on an asbestos suspension which could be deposited directly on a cathode. However, the renewal of interest in other types of cells, such as those known as the filter-press type, has directed interest towards prefabricated diaphragms. These cells, however, have been found to impose particularly stringent requirements as regards the diaphragms. On the one hand, the diaphragms are required to be very reliable and to have an increased service-life. However, it is known that the diaphragms tend to age, that is to say, their porosity, for example, diminishes with the passage of time.
This requirement as regards reliability and long service-life is also accompanied by a need for good electrical properties in view of the fact that the present trend is towards increasing current density. Diaphragms consisting essentially of asbestos fibers have, however, only led to structures whose porosity is difficult to control. Furthermore, they suffer from the same disadvantages as non-consolidated structures, namely:
swelling during electrolysis, this requiring a minimum interpolar distance; PA1 difficulties in obtaining thin deposits characterized by a low ohmic loss; and PA1 an unstable condition of the diaphragm which, after commencement of electrolysis and stabilization renders it very difficult to deal with breakdowns during operations and to effect repairs or replacements in situ. PA1 1. forming a homogeneous paste from a pore-forming charge or filler material and a latex; PA1 2. drying the paste obtained and then reducing it to powder form; PA1 3. hot-rolling a preformed product obtained from said powder; PA1 4. (fritting) sintering the membrane thus obtained; and PA1 5. finally, removing the pore-forming material.
It is for these reasons that, over a number of years, there has been a tendency to use plastic porous diaphragms constituted by membranes made of a porous material. Such materials are known per se.
It has been proposed, for example, to make diaphragms by (fritting) sintering polytetrafluoroethylene powder with a pore-forming material, (fritting) sintering being carried out at 380.degree. C. and the pore-forming material then being removed. Unfortunately, such a method results in a material which has a low proportion of cavities and which nevertheless has excessively great electrical resistance.
Also, British Pat. No. 1,081,046 discloses the idea of forming a coagulum from an aqueous polytetrafluoroethylene dispersion containing a filler substance, of then forming a sheet therefrom, and of finally removing the filler substance. This, however, does not solve the problem of shaping the coagulum. It was proposed to facilitate shaping by using a lubricant such as petroleum ether.
Unfortunately, this method suffers from the disadvantage that it does not provide reproducible results; for this reason, French Pat. No. 2,170,247 proposes the substitution of water for petroleum ether.
Numerous other disclosures are to be found relating to prior art diaphragms. Mention may be made to the following patents which employ techniques of compression preforming followed by (fritting) sintering, or techniques of coagulation of the mixture or depositing of this mixture on a support.
Thus, French Pat. No. 1,491,033 of Aug. 31, 1966, describes a process for manufacturing a porous diaphragm which consists in the sequence of: (1) mixing a solid additive in particulate form into an aqueous dispersion of polytetrafluoroethylene in the presence of particulate inorganic fillers, (2) then coagulating the dispersion, (3) placing the resulting coagulum in sheet form, and (4) finally removing the solid particulate additive from the sheet. The additive consists of starch or calcium carbonate and is removed at the end of the operation by immersing the resultant sheet in hydrochloric acid to dissolve the additive. The particulate inorganic fillers which are suitable are barium sulfate, titanium dioxide or powdered asbestos. They are used in proportions of between 40 and 70% of the weight of polytetrafluoroethylene contained in the dispersion.
British Pat. No. 943,624 of Dec. 14, 1961, proposes a method of producing a filter material which consists in mixing polytetrafluoroethylene in powder form with an eliminatable powdered material, subjecting the mixture to preforming under high pressure, and then sintering the resultant shape at a temperature which does not affect the polymer, the powdered material being eliminated either by volatilization at the sintering temperature or by the addition of solvents in which it is solubilized.
German application No. 2,140,714 of Aug. 13, 1971 describes a process of manufacturing diaphragms having a base of inorganic fibers, particularly asbestos, which are bonded by a fluorinated resin. The membrane can be obtained by impregnating a paper or fabric, or else produced by the introduction of fibers into the resin suspension and shaping in accordance with a paper-making method. The sintering is then effected under elevated pressure.
All of these foregoing prior art techniques, however, have a number of drawbacks, namely:
1. Providing flat diaphragms only, either because the use of calendering or pressing makes any other shapes impossible, or that the initial suspensions, in particular when they are coagulated, do not have sufficient properties to permit homogeneous deposits on supports of complex shape.
2. Difficulties, in the case of membranes rich in polytetrafluoroethylene, in producing membranes of satisfactory mechanical properties (permitting large flow) and of good wettability,
3. Low percentage of voids is permitted in the diaphragm structure. In order to obtain good mechanical properties and excellent conservation of the cohesion during electrolysis, the quantities of pore-forming agents used are zero or low, namely, 200-300%, or less, by weight of material. Under these circumstances, the performances in the electrolysis of sodium chloride are not truly of interest--rather large ohmic drop or low Faraday yield, resulting from the reduced porosity of the diaphragm.
Other prior art is also less than satisfactory. British Pat. No. 1,160,084, published July 30, 1969, discloses membranes and diaphragms produced from a matrix of a fluorocarbon polymer and a combustible fibrous substrate, such as of cellulose, which can be burned out of the matrix. The resulting product is porous in nature, due to the voids left by the burning of the cellulose. According to the patent, asbestos in the diaphragm is to be avoided.
British Pat. No. 1,063,244, published Mar. 30, 1967, describes a porous medium which is unsuitable for use in electrolysis cells. It is comprised of a porous base, such as of paper, having fibers, such as of asbestos, adhered to the surface, with the aid of a polymeric binder.
Bachot et al. U.S. patent application Ser. No. 469,808, filed May 14, 1974, and assigned to the assignee of the present application discloses a method which consists in forming an asbestos suspension in the presence of a surface-active agent and in adding to this suspension the latex of the fluorinated hydrocarbon resin and the pore-forming material. Although such a method gives good results and, in particular, enables a large quantity of pore-forming agent to be introduced, it still requires a certain proportion of asbestos and the use of a wet process.
It has now been found that it is possible to obtain porous membranes particularly suitable for use as diaphragms in electrolysis, using a technique which can be regarded as a dry method, and it is this method that forms the subject matter of the present invention.
It is, therefore, an object of the present invention to provide an improved dry method of producing a micro-porous membrane.
It is another object to provide an improved micro-porous membrane suitable for use in electrolysis cells.
Other objects will be apparent to those skilled in the art from the present description.